A brake actuating system of this general type is disclosed, for example, in the article ‘Electrohydraulic Brake System—The First Approach to Brake-By-Wire Technology’, SAE Papers 960991. The valve assemblies of the prior art brake actuating system which are connected upstream of the wheel brakes are configured as two-way/two-position directional control valves in slide-type construction and arranged in pairs, with the inlet valve inserted into the connection between the pressure source and the wheel brakes fulfilling a pressure-limiting function in its first switch position and releasing the connection in its second switch position. The outlet valve that is inserted into the connection between the wheel brakes and the pressure fluid supply reservoir is configured as a normally closed valve which closes the connection in its first switch position and opens it in its second switch position.
The above-mentioned publication, however, does not indicate any provisions which could eliminate or at least greatly reduce malfunctions which are caused in the prior art brake system, e.g. by gas bubbles or air bubbles.
Therefore, an object of the present invention is to improve upon an electronically controllable brake actuating system of the initially mentioned type to such effect as to greatly enhance the reliability in operation and functioning.
According to the present invention, this object is achieved by the provision of means for detecting gases or air disposed in the system.
To render the idea of the present invention more precise, the means is provided by a hydraulic arrangement which permits applying a defined pressure fluid volume to at least one wheel brake, with a pressure sensor being arranged for sensing the pressure rise which is caused in the wheel brake by the effect of the pressure fluid volume. Monitoring is thereby achieved with each actuation.
In an especially favorable improvement of the brake actuating system of the present invention, the arrangement is configured as a piston-and-cylinder unit which is interposed between the pressure source and the associated wheel brake, and a valve assembly is provided in a line that leads from the pressure source to the piston-and-cylinder unit and permits closing of the line.
Preferably, a line communicating with the pressure fluid supply reservoir and accommodating a restrictor is interposed between the valve assembly and the piston-and-cylinder unit.
In another favorable embodiment of the present invention wherein the pressure source includes a hydraulic accumulator with a movable media separating element, the hydraulic arrangement is provided by the accumulator, and there is provision of a travel sensor device for sensing the travel of the movable media separating element, the output signal of which corresponds to the defined pressure fluid volume.
In still another favorable embodiment of the subject matter of the present invention, the pressure source is arranged in a reservoir filled with pressure fluid and the accumulator housing has bores which permit the delivery of the pressure fluid into an unpressurized chamber that is defined in the accumulator housing by the movable media separating element.
To detect damage of the above-mentioned accumulator whose media separating element separates a chamber filled with a pressure fluid under high hydraulic pressure from a chamber filled with gas, a favorable aspect of the present invention arranges for the media separating element to be configured as a metal diaphragm and the pressure that prevails in the chamber filled with pressure fluid to be significantly higher than the gas pressure. The difference in pressure is preferably caused by the preload of the metal diaphragm or the additional force of a spring which acts upon the metal diaphragm. It is ensured by the mentioned provisions that in the event of leakage of the metal diaphragm an exchange of media will take place only from the pressure fluid side to the gas side.
In another favorable aspect of the object of the present invention, another possibility of sensing the leaky condition includes that the metal diaphragm has a double wall design and that a media sensor is fitted in the area between both walls which responds to pressure fluid and to gas.
The present invention will be explained in detail in the following description by way of two embodiments making reference to the accompanying drawings. In the drawings,